Salt Based Etching of Metals and Alloys for Fabricating Superhydrophobic and Superoleophobic Surfaces

ABSTRACT

A process to etch hierarchical, re-entrant texture into the surface of metals and their alloys using salt-based etching solutions. The process imbues superhydrophobic, oleophobic or superoleophobic, omniphobic or superomniphobic properties by further imparting a low surface energy coating onto the etched surfaces by chemical functionalization by low surface energy hydrophobilizing compounds.

FIELD OF THE INVENTION

The present disclosure relates to a process for surface modification of metals by developing strategies to etch surfaces of metals and alloys, and chemically modifying the etched surface to impart low surface energy properties and features, such as water and oil repellency. The current disclosure exhibits surface modification of metals through chemical approaches using different compositions and concentrations of metallic and/or non-metallic salts in aqueous, polar or non-polar solvents in the presence or absence of heat, radiation, light or electrical energy. The chemically modified metal and alloy substrates are subsequently modified with various chemical approaches that generate surface roughness and lower surface energy on the substrates, thereby enabling superhydrophobicity and oleophobicity or superoleophobicity, and often omniphobicity or superomniphobicity. The chemical etching of metals and alloys by ionic salts, such as organic or inorganic salt solutions, in the presence or absence of promoters or catalysts, and subsequent chemical functionalization strategies to create hydrophobic and oleophobic or omniphobic properties together describe a unique opportunity to improve durability, corrosion, fouling, flexibility, service life, and liquid repellency performance against water, oil, chemicals and other organic liquids.

BACKGROUND OF THE INVENTION

This section provides background information related to the present disclosure which is not necessarily prior art.

Super-repellant surfaces display low contact angle hysteresis promoting easy roll off or bouncing of the contacting liquid droplets (Patent Literature (PL) 1 and 2). To create surfaces exhibiting superomniphobic properties, the surfaces have to display super-repellent features in terms of superhydrophobicity (contact angles >150°, contact angle hysteresis <5° with water) and superoleophobicity (contact angles >150°, contact angle hysteresis <5° with low surface tension, usually γ_(LV)<30 Nm/m², with liquids such as oils and alcohols. Surfaces that exhibit contact angles greater than 150° and low contact angle hysteresis for liquids with high or low surface tension energy are said to display extreme repellency. Such super-repellent surfaces fabricated by means of chemical or physical processes are one of the most sought after materials for various automotive, aviation, materials science, biomedical, electronics, corrosion, petrochemical, and other civilian and military applications. Of late, applications have been extended to self-cleaning, non-fouling, spill-resistant fabrics and protective wears, economic consumption of energy through drag reduction and facile heat treatment, fending volcanic dusts and harsh and chemicals. Superomniphobic surfaces are those that display both superhydrophobicity and superoleophobicity (PL 1, NPL 1). The two most common parameters used to measure the extent of liquid repellency are the contact angle and the contact angle hysteresis, which is the difference between advancing and receding contact angles. A surface is considered super-repellent when it exhibits very high contact angles that are greater than 150° and very low contact angle hysteresis that are usually smaller than 5°.

Superomniphobic surfaces display high contact angles contact angle that are greater than 150° and a very low contact angle hysteresis that is usually smaller than 5° for virtually all liquids, including low surface tension liquids (PL 1). Surfaces with hierarchical scales of texture (i.e., more than one length scale of texture) display higher contact angles and lower contact angle hysteresis with a contacting liquid by entrapping air at multiple length scales, thereby reducing the solid-liquid contact area.

Surfaces that display a contact angle of greater than or equal to about 90°, optionally greater than or equal to about 95°, optionally greater than or equal to about 100°, optionally greater than or equal to about 105°, optionally greater than or equal to about 110°, optionally greater than or equal to about 115°, optionally greater than or equal to about 120°, optionally greater than or equal to 125°, optionally greater than or equal to about 130°, optionally greater than or equal to about 135°, optionally greater than or equal to about 130°, optionally greater than or equal to about 140°, and in certain aspects, optionally greater than or equal to about 145° with water or other polar liquids, and oils are considered to be “hydrophobic” and “oleophobic”, respectively.

Practical applications of hydrophobic, oleophobic, superhydrophobic, and superoleophobic surfaces are diverse, and range from stain-free clothing, spill-resistant protective wear, drag reduction, corrosion resistant coating, and chemical repellent characteristics that possess excellent mechanical, chemical and radiation durability (PL 2). These surfaces display high contact angles and low contact angle hysteresis, induced by surface roughness, hierarchical designs, and re-entrant texture of the surface, for almost all liquids, including low surface tension liquids. The basic parameter for wetting of a liquid on a smooth (non-textured) surface is the equilibrium contact angle θ, postulated from the Young's relation

${{\cos \; \theta} = \frac{\gamma_{SV} - \gamma_{SL}}{\gamma_{LV}}},$

wherein γ is me interfacial tension and S, L and V are the solid, liquid and vapor phases, respectively (PL 1, NPL 1). For interaction of a liquid droplet with a textured (including hierarchical designs) substrate, one of two configurations to minimize the droplets overall free energy can be adopted—the Wenzel state and the Cassie-Baxter state. The Wenzel state is energetically favorable, while the Cassie-Baxter state can only be metastable, for low surface tension liquids. The rational design of superomniphobic surfaces requires making the metastable Cassie-Baxter state as robust as possible.

Surfaces textured on multiple length scales are beneficial for ultra-low contact angle hysteresis (CAH), because CAH strongly depends on the liquid-air (and correspondingly the solid-liquid) interfacial area. Hierarchically textured surfaces that support a contacting droplet in the Cassie-Baxter state also display a lower contact angle hysteresis compared to surfaces that possess a single scale of texture. Contact angle hysteresis is related to energy barriers that a liquid droplet must overcome during its movement along a solid surface, and thus characterizes the resistance to the droplet movement. Lower solid-liquid contact area leads to less contact line pinning (i.e., lower resistance to droplet movement) and consequently lower contact angle hysteresis. Typically, hierarchically structured surfaces have a significantly lower solid-liquid contact area compared to surfaces that possess a single scale of texture. This can lead to significantly lower contact angle hysteresis (PL 1, NPL 1).

In addition, theoretical analysis has shown that surfaces with two scales of texture can enhance the stability of the Cassie-Baxter state by providing more locations, where the composite interface can be stable. It has been shown that the overall free energy increasingly favors the Cassie-Baxter state over the Wenzel state with an increasing number of scales of texture (NPL 1). For all these reasons, hierarchically structured surfaces possessing re-entrant texture are ideal for developing superomniphobic surfaces.

Several researchers have engineered superomniphobic surfaces utilizing hierarchically structured surfaces with re-entrant texture (PL1-2, NPL 1). However, there are two important criteria that are sometimes neglected in the literature on this subject. First, superomniphobic surfaces should display contact angles greater than 150° with low surface tension liquids and low contact angle hysteresis (Δθ*). Just reporting an apparent contact angle (θ*) or an apparent advancing (i.e., maximum) contact angle (θ_(A)*) greater than 150° for an oil or an alcohol does not adequately describe superomniphobicity. The apparent receding (i.e., minimum) contact angle (θ_(R)*), contact angle hysteresis, or roll-off angle (i.e., the minimum angle α by which the surface must be tilted relative to the horizontal for the droplet to roll-off) must also be diligently measured and reported because they are useful in estimating the ease of droplet roll-off and bouncing.

Obtaining low contact angle hysteresis (typically Δθ*<5°) and low roll-off angles (typically α<5°) for oils or alcohols is perhaps as important as the maximum achievable contact angle in qualifying a surface as superomniphobic.

Document U.S. Pat. No. 8,257,630 B2 discloses a method for fabricating a 3D (three-dimensional) structure using an aluminum foil such that the 3D structure is hydrophobic (PL 3). The method includes preparing a metal foil base by attaching the Al foil on the outer surface of a predetermined-shaped 3D structure; anodizing the Al foil base; coating a polymer material on the outer surface of the Al foil base material to form a negative replica structure; forming an outer structure by covering an outer surface of the negative replica structure with an outer formation material; and removing the metal foil base.

Recently, researchers fabricated hierarchically structured superomniphobic surfaces with a PMMA (poly(methylmethacrylate))/fluorodecyl POSS (perfluorodecyl polyhedral oligomeric silsesquioxane) coating (PL 1, NPL 1) that displayed high contact angles (θ_(A)*=155°) and ultralow contact angle hysteresis (Δθ*≦4°) even with extremely low surface tension liquids such as heptane (γ_(LV)=20.1 mN/m). Previous work has demonstrated the synthesis of fluorodecyl POSS molecules, which possess one of the lowest known surface energies (γ_(LV)≈10 mN/m). The addition of fluorodecyl POSS molecules to different polymers leads to a rapid decrease in the overall surface energy of the synthesized blends. The hierarchically structured surfaces developed in the work were fabricated by electrospinning microbeads (finer length scale texture with radius R≈3-5 μm) of 50 wt % fluorodecyl POSS and PMMA blend (γ_(LV)=10.3 mN/m) onto stainless steel wire meshes (coarser length scale texture with radius R≈50-100 μm). Both the finer and coarser length scales possess re-entrant texture. The ultralow contact angle hysteresis of the superomniphobic surfaces allowed droplets of heptane (roll-off angles α≦2°) and liquids of higher surface tension than heptane to easily roll-off and bounce.

Aluminum, Steel, Titanium and to some extent, Copper and its alloys are widely used in automotive, aerospace, aviation, shipbuilding, electronics and construction industries because of their superior mechanical properties.

Metallic and alloy surfaces can be fabricated easily by mechanical, electrical, electrochemical, chemical and physical means, but there are only handful of methods for the successful fabrication of hydrophobic, superhydrophobic, oleophobic or superoleophobic metal or alloy surfaces (NPL 3). Development of re-entrant micro or nanometer scale rough structures on the metallic surface is crucial to create oleophobic or omniphobic features.

In a recent report by Yang et al. (NPL 3), the researchers developed an easy method of fabricating superoleophobic surfaces on Al substrates by constructing re-entrant structures. The re-entrant micro/nanometer-scale structures comprise micrometer-scale, rectangular-shaped, and step-like Al structures obtained by electrochemical etching and nanometer-scale Ag grains resulting from immersion in [Ag(NH₃)₂]⁺ solution. Surface energy of the substrates is subsequently reduced by reacting with perfluorooctanoic acid (PFOA) containing —CF₃ and —CF₂— groups. The PFOA-modified micro/nanometer-scale rough structures enable the formation of a composite solid-liquid-air interface with peanut oil. These structures show good superoleophobicity with a peanut oil contact angle of 160.0±2° and sliding angle of 8°.

In the same report (NPL 3), the contact angles for water and peanut oil on PFOA-modified, polished Al surfaces were found to be 117.2° (≈θ_(water)) and 87.7° (≈θ_(oil)), respectively, which were far from being superhydrophobic and superoleophobic (≧150°). Electrochemical etching followed by developing PFOA-modified electrochemically etched Al surfaces showed extreme wettability. The contact and sliding angles of water are 167±2° and 2.5°, respectively. The contact angle of peanut oil is 145±1.5°. No sliding angle was observed for peanut oil. Scanning Electron Microscopy (SEM) images of the electrochemically etched Al surfaces obtained at the 500 mA/cm² processing current density and 6 min processing time in a 0.1 mol/L aqueous NaCl solution showed that the electrochemically etched Al surfaces are rough and covered with a large number of pits and protrusions. These protrusions can be considered as rectangular-shaped plateaus. Some step-like structures also existed on the rectangular-shaped plateaus. The electrochemically etched Al surfaces showed 1 μm to 5 μm rectangular-shaped plateaus and step-like structures.

Apart from these particular geometries, etched surfaces can also have random structures. Such surfaces may be fabricated by etching of silicon, electropolymerization and fabrication of silicone nano-filaments. Besides the attention to re-entrant structures on a micrometer-scale, these reports also highlighted the utility of hierarchical structures (i.e., multiscale surface roughness) in the enhancement of oleophobicity and the decrease of contact angle hysteresis (NPL 3, NPL 4).

Because of the difficulty in achieving superhydrophobicity and superoleophobicity, most of these surfaces have been realized with specific, sometimes non-scalable and expensive fabrication processes. Especially, on engineering materials such as, aluminum, steel, zinc, copper, titanium, chromium, nickel metals and their alloys that can be widely used in various fields, there are only a few synthetic embodiments of superoleophobic surfaces, which were synthesized using complicated and time-consuming fabrication methods, such as anodization [NPL 4]. In a recent study, Yang et al. described a method to achieve hierarchical textured surface morphology on aluminum substrates by hydrochloric acid (HCl) etching and boiling water treatment (NPL 3). This surface structure, combined with the low surface energy obtained by surface fluorination, leads to a super-repellent surface even for low surface tension liquids, such as hexadecane (surface tension=27.5 mNm⁻¹) and decane (surface tension=23.8 mNm⁻¹). Surface structures on multiple scales were controlled by the variation of etching conditions, and contact angle measurements were performed on these surfaces. Their results established the influence of surface morphology on oleophobicity of aluminum surfaces.

The etchant solutions typically used for treating metal surfaces (NPL 4), such as hydrochloric acid (HCl), nitric acid (HNO₃), sulfuric acid (H₂SO₄), hydrofluoric acid (HF), phosphoric acid (H₃PO₄), chromic acid, hydrogen peroxide (H₂O₂), potassium hydroxide (KOH), ammonium hydroxide (NH₄OH), sodium cyanide (NaCN) generally have a number of drawbacks that can include the following: (a) they are strong mineral or organic acids and bases (alkalis) that are extremely corrosive with body or skin contact, and often dangerous to handle; (b) the compounds are not easily removed with a water rinse; (c) some of these acids cannot be mixed with the most common solvents, such as water because of vigorous chemical reaction generating heat that may eventually lead to explosion without proper care—one such example is making a etchant solution from concentrated sulfuric acid and water; (d) the acids and bases are not easily disposable and extreme precaution have to be taken while using them; (e) transportation and storage of bulk mineral acids and bases is a grave concern in industries, and (f) the toxicity, handling and improper storage of these chemicals can be a safety concern. Details of other issues with commonly used etchant solutions, acids and bases are provided below.

Concentrated acids (sulfuric, phosphoric, nitric, acetic, hydrochloric, hydrofluoric and tannic acids) are corrosive to the skin, eyes, respiratory system and gastrointestinal system. Dilute acids can cause skin irritation on repeated or prolonged contact. Chromic acid is a skin sensitizer, suspect carcinogen, and oxidizer. It may cause severe kidney damage, central nervous system effects and even death if absorbed in large amounts.

Concentrated nitric acid is a strong oxidizing agent and can react explosively with other concentrated acids, solvents, etc. Nitric acid gives off various nitrogen oxide gases, including nitrogen dioxide that is a strong lung irritant and can cause emphysema.

A potent mixture of toxic nitrous oxide fumes is produced in traditional etching with nitric acid. In contact with chlorine-based cleaning products, highly toxic nerve gas (nitrogen mustard gas) can even be created. The concentrated nitric acid is a strong oxidizer which may react uncontrollably with other substances (in an exothermic reaction).

Hydrofluoric acid is highly toxic and can cause severe, deep burns which require medical attention. There is no immediate pain warning from contact with hydrofluoric acid. Mixtures of hydrofluoric acid and nitric acid are widely known as etching solutions and removing solutions for tungsten-based metals (NPL 4), but these are not preferred because silicon substrates or silicon dioxide films and glass substrates also dissolve. Another problem is that metals that are prone to corrosion, such as Al and Cu wirings, in the devices are also etched.

Mixing hydrochloric acid with potassium chlorate, for etching metals, produces highly toxic chlorine gas. Potassium chlorate is a key ingredient in many pyrotechnics, and is a potent oxidizing agent. It can react explosively with organic compounds, sulfur compounds, sulfuric acid or even dirt or clothing. On heating it can violently decompose to oxygen and potassium chloride. Storage and use are very dangerous, special precautions are required especially when mixing. (PL 3-5)

Hydrogen peroxide/ammonia/EDTA (ethylenediaminetetraacetic acid) mixtures and hydrogen peroxide/phosphate mixtures have been disclosed as means of overcoming the drawbacks of such solutions (PL 6-7, NPL 4). However, these have low titanium-based metal etching rates, while decomposition of hydrogen peroxide is rapid, making stable etching impossible. Therefore, etching solutions comprising hydrogen peroxide/phosphoric acid/ammonia mixtures have been proposed as modifications of such solutions. With such etching solutions, though the etching rate is improved, intense foaming of the etching solution causes attachment of bubbles onto the substrate surface, and etching does not proceed on the bubble-attached parts. Another problem is the low etching rate due to foaming and decomposition of the hydrogen peroxide water. Such mixtures are used with adjustment to a designated pH with ammonia, but even slight differences in pH alter the etching rate and foaming condition, and can be problematic from the viewpoint of stability of the etching conditions.

What is needed is a safe-to-handle and an easy approach for etching metals and their alloys that: (a) can, at very low levels, prevent the dissolution of metals and their alloys, including aluminum, titanium, iron, copper, zinc and other metals and their alloys (owing to their higher strength, many acids and bases can render metal surfaces passive); (b) does not chemically react to form complexes with the etchant solution; (c) can be easily rinsed from metals and alloys with water and/or a water soluble alcohol, leaving no residue; and (d) has low toxicity and does not negatively impact the biodegradability of the spent stripper solution. This present disclosure addresses and resolves these needs.

Metal salt etching process is considerably safer than acid etching because, like a battery, it is based on the principle of electrolysis (PL 8-10). Metal ions are elegantly exchanged between the poles of two kinds of metal that have an energy potential between them, unlike previous high strength, acid-base etching approaches (PL 11).

Further, the metal salt etching approach does not liberate any toxic gas unlike the acid etching process. During etching, the chemical reaction by-products are largely contained within the etching solution. The processes are not heat generating, and volatile reactions do not occur during normal use. The processes may, however, involve a degree of exposure to a chemical odor, which may be an irritant to some and should be controlled through adequate local ventilation and good air flow in the etching room. Copper sulfate solution [NPL 5] had been considered as one of the safest options among all the commonly used etching processes, mainly due to the comparatively low airborne irritation and toxicity. But recently, copper sulfate has been considered as a marine pollutant.

The Edinburgh Etch, suitable for copper, brass, and steel consists of a specific solution of ferric chloride with the addition of citric acid as a chelating agent or catalyst (NPL 5). The saline sulfate etch is designed for etching zinc, aluminum, and mild steel and consists of a copper sulfate and sodium chloride mixture in equal parts (NPL 5). The salt based etching bath with the addition of the right catalyst is formulated to obtain an effective yet safe etchant solution. For electro-etching of metals or alloys, an external current is applied to metal salt solutions (electrolyte), thus making the method homogeneous and more effective.

Oils possess much lower surface tension than water, and oil repellency requires a surface with low surface energy. The main processes involved in fabricating oleophobic and superoleophobic surfaces are the primary synthesis of rough structures possessing re-entrant textures (PL 1, NPL 1) and the subsequent lowering of the surface energy by low surface energy materials. The micro or nano roughness requirements to create superoleophobic surfaces are stringent compared to superhydrophobic surfaces.

Polished metal surfaces, fabricated by mechanical means, such as sand blasting or abrasion, exhibit hydrophilicity and oleophilicity because of their high surface free energy (PL 12-14). Lowering surface energy with low surface energy materials can reduce wettability of metal or alloy surfaces, but achieving superhydrophobicity and superoleophobicity by relying only on lowered surface energy is not feasible (NPL 2-3). Changing the wettability by constructing suitable rough structures and lowering the surface energy can be more effective.

An aqueous alkaline etching process for silicon, wherein the cleaning composition for treating the surface of silicon substrates consists of: (A) a quaternary ammonium hydroxide; and (B) a component selected from the group consisting of water-soluble acids and their water-soluble salts (PL 15). Hydrophobicity is generated by texturing the silicon substrate with a known etching solution, followed by hydrophilizing the hydrophobic surface by employing the treatment method of the invention (PL 15).

Novel methods for the texturing of photovoltaic cells has been described (PL 16), wherein texturing minimizes reflectance losses and hence increases solar cell efficiency. In one aspect, a micro stamp with the mirror inverse of the optimum surface structure is described. The photovoltaic cell substrate to be etched and the micro-stamp were immersed in a bath and pressed together to yield the optimum surface structure. In another aspect, nanoscale structures were introduced on the surface of a photovoltaic cell by depositing nanoparticles or introducing metal induced pitting to a substrate surface. In still another aspect, remote plasma source (RPS) or reactive ion etching (RIE), was used to etch nanoscale features into a silicon-containing substrate.

In another invention, etching of polymeric and metallic substrates is used to fabricate textured surfaces that promote the wetting (make more hydrophilic and oleophilic) of various liquids (PL 17). In particular, the textured surfaces were used to control the ink drawback during inkjet printing.

SUMMARY OF THE INVENTION

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The invention, which overcomes the problems mentioned above, provides an etching solution for aluminum, iron, zinc, copper and titanium-based metals and their alloys to develop re-entrant texture, reduces non-uniform etching of targets to be etched, and has a homogeneous etching rate. In addition, it provides an etching solution that requires minimal safety, handling, storage and is easy to dispose. The invention also addresses a process that imparts a thin layer of a low surface energy coating on the etched metals or alloys by chemical functionalization of the surface hydroxyl and oxide groups by hydrophobilization approaches such as, silanization, phosphatization or alkylation of the metal surfaces.

The present disclosure also describes materials having a coated surface comprising a low surface energy hydrophobilizing alkylates containing silanes, hydroxyl, carboxyl, amine, amide, ester, phosphate and sulfonate end groups that exhibit extreme wettability through tethering of fluorine atoms on the chemically modified etched metal or alloy surfaces. In certain variations, the disclosure provides a material comprising a coated surface that is superhydrophobic and oleophobic or superoleophobic, that also exhibits omniphobic or superomniphobic properties.

BENEFITS OF THE INVENTION

As described above, the method for fabricating a hydrophobic or oleophobic coating on to the etched surface of metals and alloys, according to the exemplary embodiment of the present invention, has an advantage with: (a) safety and handling of the etchants, (b) ease of use and time conservation, (c) reusability of the etchant solution, (d) fabrication of granular nano- or micro-structures on the top layer of the metal or alloy surface, (e) a single or double step chemical functionalization process for hydrophobilization of the etched metal or alloy surface with silanes, phosphates, amines and esters, (f) low fabrication cost due to simplicity of overall process and absence of using any high-priced equipment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: Schematic representation of synthetic steps for fabrication of low surface energy materials by etching and chemical functionalization.

FIG. 2: Dual magnification of etched aluminum surface.

FIG. 3: Magnification of silanized coating with hierarchical scales of texture on copper surface.

FIG. 4: Multiple magnifications of time dependent etching of an aluminum plate in Pyridine Hydrochloride solution; Insets show superoleophobic properties after silanization (θ*_(adv)=157°; θ*_(rec)=152°).

FIG. 5: Etched aluminum (top left) and TiAl alloy (top right) surface chemically modified with Heptadecafluoro-1,1,2,2-tetrahydrodecyldimethylchlorosilane exhibiting contact angle >152° and contact angle hysteresis <4° (bottom), superhydrophobic and superoleophobic properties.

FIG. 6: Etched steel and TiAl alloy, and sup erhydrophobicity features after silanization.

FIG. 7: Time-resolved videography of 1 μL water droplet bouncing on silanized etched Al and steel plates.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying figures.

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters.

As referred to herein, ranges are, unless specified otherwise, inclusive of endpoints and include disclosure of all distinct values and further divided ranges within the entire range.

By way of further background, extreme wettability can be understood in the context of the following. The primary measure of wetting of a liquid on a non-textured (or smooth) surface is the equilibrium contact angle θ from Young's relation,

${\cos \; \theta} = {\frac{\gamma_{SV} - \gamma_{SL}}{\gamma_{LV}}.}$

Here, γ refers to the interfacial tension, and S, L, and V designate the solid, liquid, and vapor phases, respectively. The solid-vapor interfacial tension (γ_(SV)) and the liquid-vapor interfacial tension (γ_(LV)) are also commonly referred to as the solid surface energy and the liquid surface tension, respectively. Non-textured surfaces that display contact angles greater than or equal to about 90° with water (or other polar liquids) are considered to be hydrophobic and surfaces that display contact angles greater than or equal to about 90° with oil (or other non-polar liquids) are considered to be oleophobic. Typically, surfaces with high γ_(sV) tend to be hydrophilic, whereas those with low γ_(SV) (such as highly fluorinated compounds) tend to be hydrophobic.

Surfaces that spontaneously approach contact angles θ greater than about 150° and low contact angle hysteresis (difference between the advancing contact angle, θ_(adv) and the receding contact angle, θ_(rec)) with water and oil are generally considered to be superhydrophobic and superoleophobic, respectively.

Superhydrophobic surfaces are those that display a contact angle of greater than or equal to about 150°, optionally greater than or equal to about 151°, optionally greater than or equal to about 152°, optionally greater than or equal to about 153°, optionally greater than or equal to about 154°, optionally greater than or equal to about 155°, optionally greater than or equal to about 156°, optionally greater than or equal to 157°, optionally greater than or equal to about 158°, optionally greater than or equal to about 159°, and in certain aspects, optionally greater than or equal to about 160° along with low contact angle hysteresis (difference between the θ_(adv) and θ_(rec) contact angles) with water or other preselected polar liquids. In certain variations, an “superhydrophobic” surface has a contact angle of greater than or equal to about 150° and less than or equal to about 180° with water or another polar liquid.

Surfaces that display a contact angle of greater than or equal to about 90°, optionally greater than or equal to about 95°, optionally greater than or equal to about 100°, optionally greater than or equal to about 105°, optionally greater than or equal to about 110°, optionally greater than or equal to about 115°, optionally greater than or equal to about 120°, optionally greater than or equal to 125°, optionally greater than or equal to about 130°, optionally greater than or equal to about 135°, optionally greater than or equal to about 130°, optionally greater than or equal to about 140°, and in certain aspects, optionally greater than or equal to about 145° with a preselected oil are considered to be “oleophobic.” Due to the low surface tension values for oils, in spite of numerous known natural superhydrophobic surfaces, there are no known, naturally-occurring, superoleophobic surfaces. A “preselected oil” is intended to include any oil or combinations of oils of interest. As discussed herein, in certain non-limiting variations, an exemplary preselected oil used to demonstrate oleophobicity is rapeseed oil (RSO).

Superoleophobic surfaces are those that display a contact angle of greater than or equal to about 150°, optionally greater than or equal to about 151°, optionally greater than or equal to about 152°, optionally greater than or equal to about 153°, optionally greater than or equal to about 154°, optionally greater than or equal to about 155°, optionally greater than or equal to about 156°, optionally greater than or equal to 157°, optionally greater than or equal to about 158°, optionally greater than or equal to about 159°, and in certain aspects, optionally greater than or equal to about 160° along with low contact angle hysteresis (difference between the advancing θ_(adv) and the receding θ_(rec) contact angles) with preselected low surface tension liquids, such as a representative oil (for example, rapeseed oil (RSO)). In certain variations a “superoleophobic” surface has a contact angle of greater than or equal to about 150° and less than or equal to about 180° with a preselected oil, like representative RSO oil.

As noted above, oleophobic and superoleophobic surfaces are generally hydrophobic and/or superhydrophobic, because the surface tension of water is significantly higher than that of oils.

Re-entrant surface texture, in conjunction with surface chemistry and roughness, can be used to engineer superomniphobic surfaces, even with extremely low surface tensions liquids such as various oils and alcohols. When a liquid contacts a porous (or textured) surface, it exhibits an apparent advancing contact angle θ* that can be significantly different from the equilibrium contact angle. If the liquid fully penetrates the porous surface, it is said to be in the Wenzel state. If the liquid does not penetrate completely, a composite (solid-liquid-air) interface forms below the drop and is considered being in the Cassie-Baxter state. In certain variations of the present disclosure, super-repellent surfaces have a surface geometry that promotes the Cassie-Baxter state. In the Cassie-Baxter state, liquid wets the porous surface up to the point where the local texture angle becomes equal to the equilibrium contact angle.

In accordance with certain aspects of the present teachings, a porous material substrate is selected to have such a desirable re-entrant surface texture (a line projected normal to the surface intersects the texture more than once), which can then be coupled with novel surface coatings to result in a low energy surface that has extreme wettability. By further design (for example, by selection or manipulation of the surface of the porous or semi-porous substrate), the hydrophobicity and oleophobicity of the surface can be preselected and tuned.

Superomniphobic surfaces are those that display low contact angle hysteresis (typically Δθ*<5°) and low roll-off angles (typically α<5°) for oils, water, polar or non-polar solvents, other chemicals (includes acids, bases and buffer solution), and optionally can include commodities such as paints, varnish, ink, ketchup, chocolate, honey, mustard sauce, liquid sauces, gravy, cooking oils etc. This low hysteresis is perhaps as important as the maximum achievable contact angle, greater than or equal to about 150°, optionally greater than or equal to about 160°, and in certain aspects, optionally greater than or equal to about 170° along with low contact angle hysteresis (difference between the advancing θ_(adv) and the receding contact angle θ_(rec)) with preselected low surface tension liquids, such as a representative oil (for example, rapeseed oil (RSO)).

In one of the embodiments, the metals used for etching process are aluminum, zinc, iron, copper and titanium, and the alloys constituted of one or more metals from aluminum, steel, copper, zinc and titanium.

In another embodiment, the metals can be solid or porous blocks of aluminum, zinc, iron, copper and titanium.

In one of the embodiments, the disclosure provides a chemical process for etching metals such as aluminum, copper, zinc, titanium, iron, nickel, chromium and their alloys with etchant solution comprising of one or more salts dissolved in liquid media or solvents to create re-entrant texture.

In yet another embodiment, the etchant (salts) reported in the disclosure can be used alone or by combining multiple salts in liquid media or solvents.

In yet another embodiment, the etchant(s) reported in the disclosure can be used with a promoter or catalyst that can be solid or liquid, in a liquid media.

In one of the embodiments, the etching solution to etch aluminum, copper, steel, titanium, zinc, nickel, chromium and their alloys, may comprise of: (i) 1 to 500 grams per liter of inorganic or organic salt, (ii) polar solvent or water, (iii) An optional solvent, preferably a polar organic solvent, and (iv) an optional promoter or catalyst that is an acid, or superacid, or base, or salt of an acid or base, or a solvent.

In another embodiment, the etchants used to fabricate re-entrant texture in the disclosure are salts of weak acid-strong base, weak base-strong acid, weak acid-weak base and strong acid-strong base in liquid media.

In yet another embodiment, the etching process may be a combination of one or more ionic salts that are inorganic and/or organic salts of weak acid-strong base, weak base-strong acid, weak acid-weak base, or strong acid-strong base. The constituents for inorganic or organic salt can be: (i) a salt comprising strong acid-weak base in a solution, or (ii) a salt comprising weak acid-strong base in a solution, or (iii) a combination of two or more salts, preferably two salts, comprising either weak acid-weak base with another salt that is a promoter or catalyst for etching, or (iv) a combination of two or more salts, preferably two salts, comprising either strong acid-strong base with another salt that is a promoter or catalyst for etching.

In another embodiment, the etching process may involve inorganic and/or organic salts containing phosphate group, such as potassium hydrogen carbonate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium dihydrogen carbonate, ammonium bromide, sodium carbonate, sodium acetate, potassium carbonate, ammonium acetate, iron chloride, iron sulfate, iron nitrate, cobalt chloride, cobalt sulfate, cobalt nitrate, pyridine hydrochloride, ammonium, sodium, potassium salts of phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, phosphovanadates, diisopropylfluorophosphate that dissociate in situ into respective cations and anions in the solvents.

In one of the embodiments, the present disclosure relates to a etching process wherein the rate of dissociation of salts can be altered by selecting a combination of solvent or buffer solution, and a promoter, or a catalyst mixed in the etching solution.

In another embodiment, the promoter or the catalyst for the etchant solution can be solid inorganic or organic salt or liquid solution.

In yet another embodiment, the optional promoter or catalyst can be a single source or combination of oxide, hydroxide, peroxide or persulfate salts of Group I and II cations, such as lithium, sodium, potassium, magnesium, calcium and barium that may alter the dissociation of salts in solutions for etching of metals and alloy substrates, in presence or absence of external energy, such as electricity, heat or pressure.

In yet another embodiment, the optional promoter or catalyst can be a single source or a combination of oxide, hydroxide and peroxide salts containing cations of ammonium, pyridinium and hydrogen that may alter the dissociation of salts in solutions for etching of metals and alloy substrates, in presence or absence of external energy, such as electricity, heat or pressure.

In one of the embodiments of the present disclosure, the rate of dissociation of salts and effect of promoter or catalyst activity may be altered by incorporating one or more sources of external energy, such as electricity, heat, mechanical agitation, pressure during the etching of the metal or alloy substrates.

In another embodiment, the rate of dissociation of salts in solutions can be altered by introducing and varying the electric current in the etchant solution during the etching process.

In another embodiment, the rate of dissociation of salts and effect of promoter or catalyst activity in solutions can be altered by introducing heat and varying temperature of the etchant solution and metal or alloy substrate during the etching process.

In another embodiment, the rate of dissociation of salts and effect of promoter or catalyst activity in solutions can be altered by introducing mechanical agitation and varying agitation speed by mechanical means in the etchant solution during the etching process.

In another embodiment, the rate of dissociation of salts and effect of promoter or catalyst activity in solutions can be altered by introducing external pressure and varying agitation speed by introducing a gas such as nitrogen, air, hydrogen or argon in the etchant solution during the etching process.

In yet another embodiment, the alteration of rate of dissociation of salts and effect of promoter or catalyst activity in solutions by electric current, heat, mechanical agitation and/or external pressure are optional, or can be applied in tandem during the etching process.

In yet another embodiment, the etching process and method may involve one or more soluble inorganic and/or organic salts in a solution phase that constitutes polar or non-polar solvents.

In one of the embodiments, the etching process may be carried out at an acidic pH between 1 and 4.5, or a basic pH between 8.5 and 14.

In one of the embodiments, washing of the etched metals and alloys will involve neutral, apolar or polar solvents, preferably with water, alcohols (ethanol, methanol isopropanol, butanol, tert-butanol), or tetrahydrofuran, hexane, heptane, acetone, acetonitrile, cyclohexane, diethyl ether etc.

In yet another embodiment, the etched metals such as aluminum, zinc, iron, copper and titanium or their alloys are analyzed using microscopy and has micro- or nano-roughness on the surface.

In yet another embodiment, the disclosure provides a process, wherein the hierarchical, re-entrant nano- and micro-textures are fabricated by subjecting the etched metals or alloys possessing re-entrant texture on a single length scale to 40° C. to 200° C. in boiling solvents, preferably water and/or steam, with or without external pressure and agitation for a time interval between 5 minutes to 24 hours.

In one of the embodiments, the disclosure provides a process for generating micro- or nano-patterns on etched metal or alloy surface.

In another embodiment, the uniform micro- or nano-granular morphology on the metals or alloys create surface roughness for functionalization with organic or inorganic functional groups.

In another embodiment, the disclosure provides a process for functionalizing the nano- or micro-patterned etched surfaces of the metals or alloys by chemical modification.

In yet another embodiment, the chemical functionalization of the etched metals or alloys is carried out at −10° C. to 200° C., preferably between 15° C. and 110° C.

In another embodiment, time taken to etch the metals or alloys is about 1 minute to 48 hours, preferably 1 hour to 12 hours.

In yet another embodiment, the disclosure provides a salt-based etching process at temperature ranging from −10° C. to 200° C. for aluminum, copper, zinc, titanium, iron, chromium, nickel and their alloys.

In one of the embodiments, the etchant solution can be reused and recycled for the etching process and chemical modification of metals or their alloys.

In yet another embodiment, the disclosure provides a salt-based etching process, wherein the solution can reused for as much as 50 times for etching metals and alloys.

In yet another embodiment, the disclosure provided a process, wherein, the chemically treatment of the etched metal or alloy surface exhibits both hydrophobicity or superhydrophobicity and oleophobicity or superoleophobicity, having a contact angle hysteresis less than or equal to about 5° for water and preselected oil, organics, chemicals and commodity materials such as coffee, chocolate, syrup, cleaner solutions etc.

In yet another embodiment, the disclosure provides a material having a coated nano- or micro-patterned surface that is both superhydrophobic and superoleophobic, and often display superomniphobic features, having a first apparent advancing dynamic contact angle of greater than or equal to about 150° for water and preselected oil, respectively. In certain variations, the etched metal surface is further treated with low surface energy fluoroalkyl silanes, alkyl silanes, alkyl phosphates, alkyl sulfonates, alkyl bound inorganic salts such as phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, phosphovanadates, and fluoroalkyl functional group or fluorine atoms bound acids, amines, alcohols, alkoxy, esters, acrylates, halogenated silane molecules having a surface tension of less than or equal to about 20 mN/m reacted with hydroxyl, oxide, silane, halogen, amine or alkoxy groups made available synthetically on the porous metals or alloys.

In yet another embodiment, the disclosure describes a method for forming a thin layer coating on a surface having a predetermined wettability. The method comprises reacting a low surface energy fluoroalkyl silanes, alkyl silanes, alkyl phosphates, alkyl sulfonates, alkyl bound inorganic salts such as phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, phosphovanadates, and fluoroalkyl functional group or fluorine atoms bound acids, amines, alcohols, alkoxy, esters, acrylates, halogenated silane molecules having a surface tension of less than or equal to about 25 mN/m attached to or on the etched metallic or alloy surface through chemical functional groups.

In yet another embodiment, the disclosure provides a material having coated surface that is superhydrophobic or superoleophobic, wherein the etched metal or alloy surface-binding chemical functional groups can be alkyl, methoxy, ethoxy, amine, carboxyl, hydroxyl, sulfonic, halogen, thiocyanate, cyanate, thiol, or phosphate present in the low surface energy molecule.

In yet another embodiment, the disclosure provides etched metals or their alloys having chemically functionalized nano- or micro-textured surface that is superhydrophobic, having a first apparent advancing dynamic contact angle greater than or equal to about 150° for polar liquids such as water, acid, base, or that is superoleophobic having a first apparent advancing dynamic contact angle greater than or equal to about 150° for a non-polar (oil, alkanes, alkenes, alkynes, solvents, chemicals, organics, food-based commodities etc.).

In another embodiment, the disclosure provides a method for chemical functionalization of the etched metals or alloy surfaces that imparts surfaces with hydrophobic, superhydrophobic, oleophobic, superoleophobic, omniphobic or superomniphobic properties.

In another embodiment, the disclosure provides a method, wherein the surface roughness created on the metals or alloys is utilized for functionalization with organic or inorganic functional groups.

In yet another embodiment, the disclosure provides a method, wherein the etching process of the metals or alloys is carried out at temperatures between −10° C. and 100° C., preferably between 15° C. and 80° C.

In another embodiment of the present disclosure, the chemical functionalization of the etched metals or alloys to generate low surfaces energy with hydrophobic, oleophobic or omniphobic properties is carried out by tethering processes, such as silanization, silylation phosphatization, parkerization, carboxylation, amination, sulfonation, carbonation, esterification, hydroxylation, amidation, thiolation, azolation.

In another embodiment, the chemical functionalization of the etched metals or alloys may be carried out in 1 minute to 72 hours, preferably between 8 hours and 48 hours.

In another embodiment, the chemical functionalization of the etched metals or alloys can be carried out at 110° C. to 200° C. with steam.

In another embodiment, the hierarchical, re-entrant nano- and micro-textures are fabricated by subjecting the etched metals or alloys to 40° C. to 200° C. in boiling solvents, preferably water and/or steam, with or without external pressure and agitation for a time interval between 5 minutes to 24 hours.

In yet another embodiment, the hierarchical, re-entrant nano- and micro-textures are fabricated by etching in different salt solutions with varying concentrations, from 0.1 M to 10 M. The different salt solutions may be at different temperatures.

In one of the embodiments, the chemically functionalized etched metals or alloys are rinsed and washed with polar, apolar or neutral solvents, preferably tetrahydrofuran, or an alcohol, or deionized water at pH 7 at the end of the functionalization process.

In another embodiment, the chemical functionalization imparts a thin layer of low surface energy (γ_(LV)) coating on the etched metal or alloy surface; with a thickness about 5 angstrom to 1 micron, preferably 2 nanometer to 500 nanometer.

In another embodiment, the thin layer low surface energy thin layer coating can impart properties such as superhydrophobic or superoleophobic, or superomniphobic.

In yet another embodiment, the hydrophobic and/or oleophobic coating on the etched metals or alloys can show superhydrophobic and/or superolephobic properties that repel water, oils, liquids and food-based commodities such as ketchup, chocolate, sauce, wine, vinegar, honey.

In yet another embodiment, the etched metals or alloys can exhibit superhydrophobic and/or superoleophobic properties with contact angles from 150° to 180°, preferably between 150° and 179° with water or various oils.

In another embodiment, the low surface energy etched metals or alloys may exhibit omniphobic feature, which can impart both hydrophobic or oleophobic properties with respect to water and oil contact angles respectively, and the contact angle can range from 100° to 179°, preferably between 120° to 179° with water or various oils.

In another embodiment, the superhydrophobic or superoleophobic, or superomniphobic coatings can be made transparent or visually transparent, translucent or opaque.

In another embodiment, the superhydrophobic or superoleophobic etched metals may exhibit acid and base resistance, and liquid repellent properties.

In one of the embodiments of the present disclosure, the etched metal or alloy surfaces can be modified by means of chemical functionalization methods and approaches such as, silanization, carboxylation, hydroxylation, amidation, amination, sulfonation, phosphatization, parkerization, halogenation including fluorination, alkylation and alkoxylation to develop lower surface energy substrates.

In another embodiment, the predetermined wettability may be selected to be hydrophobic or oleophobic, having the first apparent advancing dynamic contact angle of greater than or equal to about 150° for water, and/or having the second apparent advancing dynamic contact angle of greater than or equal to about 150° for a preselected oil, wherein the low surface energy fluoroalkyl silane comprises heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane, or combinations thereof, and the reacting of the low surface energy fluoroalkyl silane with oxygen groups, carbon groups, carboxyl groups, nitride groups, carbide groups, and hydroxyl groups on the pristine or etched metal or alloy surface occurs at room temperature for a duration of greater than or equal to about 1 hour to less than or equal to about 3 days until greater than or equal to about 60% of the chemical functional groups have reacted or physisorbed with the low surface energy fluoroalkyl silane.

In yet another embodiment, the predetermined wettability may be selected to be hydrophobic or olephobic, having the first apparent advancing dynamic contact angle of greater than or equal to about 150° for water, and/or having the second apparent advancing dynamic contact angle of greater than or equal to about 150° for a preselected oil, wherein the low surface energy fluoroalkyl silane comprises heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane, or combinations thereof, and the reacting of the low surface energy fluoroalkyl silane with oxygen groups, carbon groups, carboxyl groups, nitride groups, carbide groups, and hydroxyl groups on the pristine or etched metal or alloy surface occurs at room temperature under vacuum or nitrogen or argon gas atmosphere for a duration of greater than or equal to about 1 hour to less than or equal to about 3 days until greater than or equal to about 40% of the chemical functional groups have reacted or physisorbed with the low surface energy fluoroalkyl silane.

In one of the embodiments, the low surface energy fluoroalkyl silane can be selected from a group consisting of: heptadecafluoro-1,1,2,2-tetrahydrodecyltriethoxysilane, heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrodecyldimethylchlorosilane, heptadecafluoro-1,1,2,2-tetrahydrodecylmethyldichlorosilane, tridecafluoro-1,1,2,2-tetrahydrooctyltriethoxysilane, tridecafluoro-1, 1,2,2-tetrahydrooctyltrichlorosilane, heptadecafluorodecyltriethoxysilane, heptadecafluorodecyltrichlorosilane, heptadecafluorodecyltrimethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane, 3,3,3-trifluoropropylmethyldimethoxysilane, 3,3,3-trifluoropropylmethyldichlorosilane, 3,3,3-trifluoropropyldimethylchlorosilane, bis(trifluoropropyl)tetramethyldisilazane, nonafluorohexyltrichlorosilane, nonafluorohexyltriethoxysilane, nonafluorohexylmethyldichlorosilane, nonafluorohexyldimethylchlorosilane, 3,3,3-trifluoropropyltrichlorosilane, nonafluorohexyltrimethoxysilane, nonafluorohexyl tris(dimethylamino)silane, tidecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane, tridecafluoro-1,1,2,2-tetrahydrooctylmethyldichlorosilane, tidecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane, heptadecafluoro-1,1,2,2-tetrahydrodecyl trimethoxysilane, 3-heptafluoroisopropoxypropyltrichlorosilane, hexadecafluorododec-11-Enyl-1-trimethoxysilane, a mixture of perfluorododecyl-1H,1H,2H,2H-triethoxysilane and perfluorotetradecyl-1H,1H,2H,2H-triethoxysilane, 1,8-bis(trichlorosilylethyl)hexadecafluorooctane.

In one of the embodiments, the predetermined wettability is selected to be hydrophobic or oleophobic, having the first apparent advancing dynamic contact angle of greater than or equal to about 150° for water, and/or having the second apparent advancing dynamic contact angle of greater than or equal to about 150° for a preselected oil, wherein the low surface energy fluoroalkyl acids comprise of acidic groups, such as carboxylic (—COOH) functional groups and fluoro-carbon pendant atoms bound the molecule comprising of —COOH group at a random position attached to the carbon chain of the molecule. The fluoroalkyl acids comprising of a long chain carbon structure comprising of 5 to 18 carbon atoms chain length can be Heptadecafluorooctanesulfonic acid solution, ammonium, potassium or sodium salt of Heptadecafluorooctanesulfonic acid, Nonafluoro-1-butanesulfonic acid, ammonium, potassium or sodium salt of Nonafluoro-1-butanesulfonate, Tetrabutylammoniumnonafluorobutanesulfonate, Pentadecafluorooctanoic acid, Perfluorodecanoic acid, Perfluoroheptanoic acid and ammonium, lithium, sodium or potassium salt of Heptadecafluoroisooctanesulfonic acid.

In another embodiment, the predetermined wettability is selected to be hydrophobic or oleophobic, having the first apparent advancing dynamic contact angle of greater than or equal to about 150° for water, and/or having the second apparent advancing dynamic contact angle of greater than or equal to about 150° for a preselected oil, wherein the low surface energy fluoroalkyl acids comprise of acidic, such as carboxylic (—COOH) and sulfonic (—SOOOH) functional groups, and fluoro-carbon pendant atoms bound the molecule. The fluoroalkyl phosphates can be polyfluoroalkyl phosphates such as 1-Decanolheptadecafluorodihydrogenphosphate.

In yet another embodiment, the predetermined wettability is selected to be hydrophobic or oleophobic, having the first apparent advancing dynamic contact angle of greater than or equal to about 150° for water, and/or having the second apparent advancing dynamic contact angle of greater than or equal to about 150° for a preselected oil, wherein the low surface energy alkyl silanes, fluroalkyl or alkyl phosphates, fluoroalkyl or alkyl sulfonates, fluoroalkyl or alkyl bound inorganic salts such as phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, phosphovanadates, and fluoroalkyl functional groups or fluorine atoms bound acids, amines, alcohols, alkoxy, esters, acrylates, halogenated silane molecules.

In another embodiment, the surface of the porous, semi-porous or solid metal or alloy material may comprise of non-ionic, ionic, covalent, coordinate covalent bonded functional groups involving oxygen, carbon, carboxyl, silicon, phosphorus, nitride, carbide, sulfide, cyanide, thiol, hydroxyl and/or other chemical functionalities.

In another embodiment, the solvents used for etching, rinsing and chemical modification may consist of liquid or vapor phase water, tetrahydrofuran, hexafluorobenzene, ethanol, methanol, isopropanol, butanol, tert-butanol, acetone, acetonitrile, hexane, cyclohexane, diethyl ether, heptane, toluene, chloroform, methylene chloride, or a combination of 3,3-Dichloro-1,1,1,2,2-pentafluoropropane and 1,3-Dichloro-1,1,2,2,3-pentafluoropropane, and other polar or non-polar organic solvents.

In yet another embodiment, the etching process for the metal or alloys and post treatment of the metal or alloys substrates by means of chemical functionalization may impart features such as transparent, visually transparent, translucent, opaque, glossy, semi-glossy, nano-granular, matte finish, with or without a color.

In yet another embodiment, the low surface energy coating on the etched metal or alloy surface may comprise of a low surface energy fluoroalkyl silane having a surface tension of less than or equal to about 15 mN/m.

In another embodiment, a single step or multiple-step process may involve etching by by sand blasting, abrasion using sand paper or file to generate nano- or micro-patterned rough surface, followed by boiling in a solvent or combination of solvents, or etching in a salt solution followed by a procedure that may involve spraying, layer-brushing, painting, spin coating, dip coating, lithography on the etched metal or alloy surfaces to develop low surface energy thin layer coating.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

To carry out experiments for measuring contact angles, advancing angles, receding angles and contact angle hysteresis, measurements may be made by advancing and receding a single droplet of liquid (≈2 μL) from a 2 mL micrometer syringe. Averages from at least five independent measurements will be considered. The surface tension of probe liquids are evaluated using the pendant drop method with ±5% error limits.

To validate surface morphologies pre- and post-coating or dipping experiments, surfaces may be imaged using a scanning electron microscope (SEM), atomic force microscope (AFM) and transmission electron microscope (TEM) to measure thin layers.

DETAILED DESCRIPTIONS OF DRAWINGS

The present invention will now be explained in greater details based on based on examples and comparative examples with reference to the attached figures, with the understanding that the invention is related to these descriptions. With regard to the figures, like reference numerals refer to like parts.

FIG. 1 shows a generic schematic of the process for the present invention, as shown with aluminum 100 and titanium aluminum alloy (TiAl) 200 in FIG. 1-A. The current disclosure describes a process that involves etching of metal or alloy surface (step 1), treating etched surface in boiling water (step 2) for fabricating nano- and micro-patterned surfaces (step 3), and chemical functionalization of the etched materials (step 4) to fabricate low surface energy surfaces, thereby developing oleophobic, hydrophobic or omniphobic surfaces of octane 10, water 20, and methanol 30. A magnification of etched aluminum 110 and etched TiAl 210, after step 3, is shown in FIG. 1-B. The results of chemical functionalization (step 4) are shown in FIGS. 1-C and 1-D of silanized etched aluminum 120 and silanized etched TiAl 220, respectively.

Descriptions for the process of etching metals and alloys include the following: Aluminum, NiAl alloy, Invar alloy, Steel, Titanium, Copper, Brass plates (4×2×0.2 cubic inch, 99.5%) were cleaned ultrasonically with acetone and ethanol to get rid of any grease. The cleaned metal or alloy plates were etched to create re-entrant texture in separate experiments with 0.5 (M), 1.0 (M), 1.5 (M), 2.0 (M) and 2.5 (M) solutions [where, ‘M’ stands for molar] of potassium hydrogen carbonate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium dihydrogen carbonate, ammonium bromide, sodium carbonate, sodium acetate, potassium carbonate, ammonium acetate, iron chloride, iron sulfate, iron nitrate, cobalt chloride, cobalt sulfate, cobalt nitrate, pyridine hydrochloride, ammonium, sodium, potassium salts of phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, phosphovanadates, diisopropylfluorophosphate at room temperature for 10 minutes to 72 hours. After being rinsed with deionized water, the samples were dried in a flow of air or nitrogen at 25° C. FIGS. 2 to 7 represent etching of different metals and alloys using the current method in the disclosure.

Descriptions for the process of generation of nano-surface roughness include the following: Etched metal and alloys were subjected to boiling water to generate nano-roughness onto the surfaces (prior to chemical functionalization). This step can generate surface hydroxyl or oxide groups upon drying. FIG. 2 shows magnification of etched aluminum 110. FIG. 2-A is at 200 um scale and FIG. 2-B is at 2 um scale. FIG. 4 shows time dependent etching of aluminum 100 at multiple magnification scales. FIGS. 4-B, F, and J show the surface at 500 μm scale at 0, 8, and 16 minutes of processing, respectively. FIGS. 4-C, G, and K show the surface at 50 μm scale at 0, 8, and 16 minutes of processing, respectively. FIGS. 4-D, H, and L show the surface at 50 μm scale at 0, 8, and 16 minutes of processing, respectively. FIGS. 5-A and 5-B show etched aluminum 110-A and etched TiAl 210-A, respectively. FIG. 6-A shows a magnification of etched steel 410 and FIG. 6-D shows a magnification of etched TiAl 210-B.

Descriptions for the process of chemical functionalization include the following: Followed by the etching and nano-roughness generation procedures, the hierarchically-textured etched metal or alloy substrates were subjected to chemical functionalization by immersing in various concentrations ranging from 0.01 M to 0.1 M Heptadecafluoro-1,1,2,2-tetrahydrodecyldimethylchlorosilane, Perfluorodecanonic acid, Heptadecafluoro-1,1,2,2-tetrahydrodecylmethyldichlorosilane, Heptadecafluorooctanesulfonic acid solution (40% in water), Heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, Perfluoropentanoic acid, Heptadecafluoro-1,1,2,2-tetrahydrodecyltriethoxysilane, Pentadecafluorooctanoic acid, 1-Decanolheptadecafluorodihydrogenphosphate solutions for 1 to 15 hours at 25° C., and subsequently rinsing with deionized water, and drying in nitrogen or air to fabricate low surface energy (γ_(SV)) materials. Figures listed represent chemical functionalization of the etched materials to impart low surface energy thin layer coating on the materials rendering superhydrophobic, superoleophobic or superomniphobic properties. FIG. 3 shows a 5 μm scale magnification of silanized coating 40 with hierarchical scales of texture. FIG. 6-B shows a magnification of silanized steel 420 and FIG. 6-E shows silanized etched TiAl 220-B.

Descriptions for the process of measurements and characterization include the following: To carry out experiments to determine contact angles, advancing angles, receding angles and contact angle hysteresis of the low surface energy surfaces fabricated by the present disclosure, tests and experiments were carried out by advancing and receding a single droplet of liquid (≈2 μL) from a 2 mL micrometer syringe using high speed camera facilities. Averages from five independent measurements were considered to determine the final data. The surface tension of probe liquids were evaluated using the pendant drop method with ±5% error limits. The figures described exhibit contact angle measures using water 20, octane 10 and methanol 30. FIGS. 4-A, E, and I insets demonstrate superoleophobic properties after silanization on aluminum 100 for octane 10 and methanol 30 at 0, 8, and 16 minutes, respectively. FIGS. 5-C and 5-D demonstrate the silanized etched properties on aluminum 100-A and TiAl 210-A, respectively, with octane 10, water 20, and methanol 30. FIG. 6-C demonstrates the material properties of steel 410 with octane 10, water 20, and methanol 30.

FIGS. 7-A and 7-B are a time-frozen, high speed video images of a water droplet 20 bouncing on silanized etched aluminum 120 and steel 420, respectively, to demonstrate the low surface energy materials developed using the methodology described in the current disclosure. To validate surface morphologies pre- and post-etching and chemical functionalization experiments, chemically modified etched metals and alloys with low energy surfaces were imaged using a scanning electron microscope (SEM), atomic force microscope (AFM) and transmission electron microscope (TEM) to determine thin layer and morphology. The attached figures represent various images of the etching, surface roughness, chemically functionalized surfaces and porous materials using surface imaging facilities, as described previously. 

What is claimed is:
 1. A method for creating a hierarchical re-entrant texture of a metallic or metallic alloy surface comprising: etching the metal surface with an etching solution having 1 to 500 grams per liter of an at least one salt including an inorganic salt or organic salt, and water.
 2. The method of claim 1 wherein the at least one salt is a weak acid-strong base, weak base-strong acid, weak acid-weak base, or strong acid-strong base salt.
 3. The method of claim 2 wherein the etching solution is comprised of a salt having a strong acid-weak base in solution, or having a weak acid-strong base in solution, or a combination of two or more salts having either a weak acid-weak base with another salt that is a promoter or catalyst for etching, or a combination of two or more salts having a first salt of a strong acid-strong base and a second salt that is a promoter or a catalyst.
 4. The method of claim 1 wherein the at least one salt is operable to dissociate a cation and an anion in the etching solution and wherein the at least one salt contains potassium hydrogen carbonate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium carbonate, sodium dihydrogen carbonate, ammonium bromide, sodium carbonate, sodium acetate, potassium carbonate, ammonium acetate, iron chloride, iron sulfate, iron nitrate, cobalt chloride, cobalt sulfate, cobalt nitrate, pyridine hydrochloride, ammonium, sodium, potassium salts of phosphotungstic acid, phosphomolybdic acid, phosphosilicic acid, or phosphovanadates, diisopropylfluorophosphate.
 5. The method of claim 1 further comprised of a promoter or catalyst wherein the promoter or catalyst is operable to alter dissociation of the at least one salt in the etching solution and wherein the at least one salt is an oxide, hydroxide, peroxide or persulfate salt of Group I and II cations including lithium, sodium, potassium, magnesium, calcium or barium.
 6. The method of claim 1 wherein the process is further comprised of adding external energy including electricity, heat, mechanical agitation or pressure.
 7. The method of claim 1 further comprised of a promoter or catalyst wherein the promoter or catalyst is operable to alter dissociation of the at least one salt in the etching solution and wherein the at least one salt is an oxide, hydroxide and peroxide salts containing cations of ammonium, pyridinium and hydrogen.
 8. The method of claim 1 wherein the etching solution further comprises a solvent whereby the solvent has at least one of a polar solvent or a non-polar solvent.
 9. The method of claim 1 wherein the etching solution has an acidic pH.
 10. The method of claim 1 wherein the etching solution has an acidic pH between 1 and 4.5, or a basic pH between 8.5 to
 14. 11. The method of claim 1 further comprised of a step wherein the surface is washed with at least one solvent being a neutral, apolar or polar solvent including water, ethanol, methanol isopropanol, butanol, tert-butanol, tetrahydrofuran, hexane, heptane, acetone, acetonitrile, cyclohexane, or diethyl ether.
 12. The method of claim 1 wherein the metallic or metallic alloy material is comprising at least one metal including aluminum, zinc, iron, copper and titanium.
 13. The method of claim 1 wherein the metal etching step has a time duration from 1 minute to 48 hours.
 14. The method of claim 1 wherein the metal etching step is at a temperature from −10° C. to 100° C.
 15. The method of claim 6 wherein the adding of external energy is operable to vary the at least one salt rate of dissociation and effect of promoter or catalyst activity by varying at least one parameter from electrical current, from temperature of the etching solution, from temperature of the surface, from agitation speed of mechanical agitation in the etching solution, from introducing external pressure and introducing a gas including nitrogen, air, hydrogen or argon in the etching solution.
 16. The method of claim 1 wherein a rate of etching the metal surface is operable by varying the rate of salt dissociation by further comprising the etching solution with the promoter or catalyst and at least one of the solvent or a buffer solution in the etching solution.
 17. The method of claim 1 wherein the optional or the catalyst in the etching solution is a solid inorganic salt, solid organic salt, or liquid solution.
 18. The method of claim 1 further comprised of a chemical functionalization step of the etched surface operable to generate a low energy surface on the etched surface with hydrophobic, oleophobic or omniphobic properties by a tethering processes including silanization, phosphatization, parkerization, carboxylation, amination, sulfonation, carbonation, esterification, hydroxylation, thiolation amidation, azolation, silylation, halogenation including fluorination, alkylation or alkoxylation.
 19. The method of claim 19 wherein the chemical functionalization step possesses a re-entrant surface texture.
 20. The method of claim 19 wherein re-entrant texture is a hierarchical texture having re-entrant nano- and micro-textures by further comprising the step of adding the solvent into the etching solution and the step of boiling the etching solution at a temperature from 40° C. to 200° C.
 21. The method of claim 20 wherein the solvent is water or steam.
 22. The method of claim 1 further comprised of a chemical functionalization step of the etched surface operable to generate a low energy surface with hydrophobic, oleophobic or omniphobic properties and washing step with the solvent including a polar, apolar or neutral solvent comprised of at least solvent selected from ethanol, methanol isopropanol, butanol, tert-butanol, tetrahydrofuran, hexane, heptane, acetone, acetonitrile, cyclohexane, or diethyl ether.
 23. The method of claim 1 wherein the surface is superhydrophobic or superoleophobic, or superomniphobic.
 24. The method of claim 1 further comprised of a step applying a low surface energy and thin layer coated on the etched surface operable to obtain at least one of superhydrophobic or superoleophobic property having a contact angle from 150° to 180° with at least one of water or oil.
 25. The method of claim 1 further comprised of a step applying a low surface energy and thin layer coated on the etched surface operable to obtain at least one of omniphobic property having a contact angle from 100° to 179° with water and oil.
 26. The method of claim 1 further comprised of at least one additional step including etching by sand blasting or abrasion using sand paper or file to generate nano- or micro-patterned rough surface, followed by an additional step of boiling in the solvent, or after the etching step in a salt solution followed by an additional step of spraying, layer-brushing, painting, spin coating, dip coating, or lithography on the etched surface operable to develop a low surface energy thin layer coating.
 27. The method of claim 1 wherein the etched surface has a predetermined wettability selected to be hydrophobic or oleophobic, has an initial non-reacted chemical functional groups, having a first apparent advancing dynamic contact angle of greater than or equal to 150° for water, and having a second apparent advancing dynamic contact angle of greater than or equal to 150° for a preselected oil, and an additional step of applying a low surface energy coating of a fluoroalkyl silane comprising heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane, heptadecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane, or combinations thereof, and an additional subsequent step of reacting the low surface energy fluoroalkyl silane with oxygen groups, carbon groups, carboxyl groups, nitride groups, carbide groups, and hydroxyl groups on the etched surface for a step duration of time until greater than or equal to 60% of the initial non-reacted chemical functional groups are reacted or physisorbed by the fluoroalkyl silane.
 28. The method of claim 1 wherein the solvent used for at least one step of etching, rinsing or chemical functionalization is comprised of a liquid or vapor phase water, tetrahydrofuran, hexafluorobenzene, cyclohexane, or a combination of 3,3-Dichloro-1,1, 1,2,2-pentafluoropropane and 1,3-Dichloro-1,1,2,2,3-pentafluoropropane, toluene, chloroform, methylene chloride, and other polar or non-polar organic solvents. 